It is common to provide a sample of an image to a customer for approval prior to printing a large number of copies of an image using a high volume output device such as a printing press. The sample image is known as a “proof.” The proof is used to ensure that the consumer is satisfied with, among other things, a color of the image.
It is not, however, cost effective to print the proof using high volume output devices of the type used to print large quantities of the image. This is because it is expensive to set up high volume output devices to print the image. Accordingly, it has become a practice in the printing industry to use digital color printers to print proofs. Digital color printers render color prints of images that have been encoded in the form of digital data. This data includes code values indicating the colors to be printed in the image. When the color printer generates a printed output of an image, it is intended that the image recorded on the printed output will contain the exact colors called for by the code values in the digitally encoded data.
In practice, it has been found that colors printed by digital color printers do not always match colors printed by high volume output devices. One reason for this is that variations in ink, paper and printing conditions can cause the digital color printer to generate images with colors that do not match the colors produced by the high volume output device using the same values. Therefore, a proof printed by the digital color printer may not have colors that match the colors printed by the high volume output device.
Accordingly, digital color printers have been developed that can be color adjusted so that they can mimic the performance of high volume output devices. Such adjustable color printers are known in the industry as “proofers.”
Color calibration adjustments are used to modify the operation of the proofer so that the proofer prints the colors called for in the code values of the images to be printed by the proofer. These adjustments are necessary to compensate for the variations in ink, paper, and printing conditions that can cause the colors printed by the proofer to vary from the colors called for in the code values.
To determine what color calibration adjustments must be made, it is necessary to determine how the proofer translates code values into colors on the printed image. This is done by instructing the proofer to print a calibration test image or so-called “color chart.” The calibration test image includes a number of colorant-combination patches. Each colorant-combination patch contains the color printed by the proofer in response to a particular code value.
Typically, a manual stand-alone calibration device is used to measure the colors in the test image. The measured color of each colorant-combination patch is converted into a color code value and is compared against the original “color chart” code value associated with that patch. Thereafter, comparisons are used to determine what adjustments must be made to the proofer to cause the proofer to print the desired colors in response to the particular color code values.
Color management adjustments are used to modify the operation of the proofer so that the image printed by the proofer will have an appearance that matches the appearance of the same image as printed by the high volume output device. The first step in color management is to determine how the high volume output device converts color code values into printed colors. This is known as “characterization.” The result of such a characterization process is a “color profile.”
It is recognized that calibration adjustments are based upon objective measurements of the color and tone characteristics of test images printed by the proofer and high volume output device. The most accurate device for measuring color for calibration and confirmation purposes is a spectrophotometer. A spectrophotometer measures the reflectance and/or transmittance of an object at a number of wavelengths throughout the visible spectrum. More specifically, a spectrophotometer exposes a test image to a known light source and then analyzes the light that is reflected by the test image to determine the spectral intensity. A typical spectrophotometer is capable of measuring a group of pixels in an image. It includes an apparatus that measures the light that is reflected by a portion of an image at a number of wavelengths throughout the visible spectrum to obtain data that represents the true spectral content of the reflected light.
Currently most proofing systems that utilize wide format inkjet printers, employ calibration technologies to ensure that a given inkjet printer produces output colors that closely match defined goal colors. It is thereby ensured that a plurality of printers of the same type will reproduce the goal colors quite closely. Many commercial proofing calibration implementations use the common approach of calibrating at certain times, e.g. Monday mornings or upon failure. Some software packages offer automated execution based on scheduling. This approach has the fundamental disadvantage that color can drift or change significantly between calibrations. Furthermore, if an unrepresentative print is used as the input for calibration routine, it may result in skewing the color output to undesired values. This undesirable shift could only be identified by verifying the output of a calibrated printer against the goal colors.
In general inkjet printers do not produce perfect proof-to-proof color consistency. In addition to short term noisy behavior, slow drifting and step color shifts also occur due to environmental changes such as temperature or humidity, ink variations due to, for example lot variation, media changes due to, for example, lot-to-lot variability, and hardware changes such as printhead replacement.
The performance variation of inkjet printers is also typically not tracked. The absence of such data makes it difficult to establish routines for adjusting the printer, or the data sent to the printer, so as to render colors more consistently.
Furthermore the current calibration embodiments from software vendors still require ongoing manual interventions to perform and monitor the state of the printing system and identify when the calibration should be redone.
Recently an increasing number of wide-format inkjet printers offer built-in spectrophotometers, which provide sufficiently accurate color measurements to allow the printers to be used as proofing systems. Examples of such systems include the B2 printer from Dupont-Nemours, the Veris system from Kodak, and the Z2100 and Z3100 systems from Hewlett-Packard. These devices lend themselves to better automation and reduced user intervention. In addition, different vendors now offer software packages supporting calibration of the printer with the built-in spectrophotometer as well as verification of color output and support for the measurement of color profiling charts.
Despite all the advantageous technological developments described in the foregoing, the actual adjustment of printers at present in the market place to ensure that an inking provides a desired goal color is still based on a simple point by point comparison of rendered colors with goal colors. In the prior art, extensive color models, based on previously stored data or on data collected during the calibration process, have been used to predict adjustments needed to be made to give consistent color rendering on the proofer. Unfortunately the coarse sampling of the color space leads to inaccurate color adjustments as it does not adequately reflect actual local color behavior. Against this background there remains a clear need for a method to automatically and accurately calibrate a proofing printer which allows for more rapid local variation in the color space.